Lascano, G. J., Heinrichs, A. J. and Tricarico, J. M. 2015. Saccharomyces cerevisiae live culture affects rapidly fermentable carbohydrates fermentation profile in precision-fed dairy heifers. Can. J. Anim. Sci. 95: 117-127. The experimental objective was to determine the dose effect of live yeast culture (YC) on rumen fermentation profiles and microbial total cell concentrations in precision-fed dairy heifers exposed to different rapidly fermented carbohydrates diets. A split-plot design with starch level as the whole plot and YC dose as sub-plot was administered in a four-period (21 d) 4×4 Latin square balanced for carryover effects. Eight Holstein heifers were allocated to two starch treatments (28% starch: HS; 17% starch: LS) and to a sequence of YC doses (0, 10, 30, and 50 g d^-1). Total volatile fatty acid concentration was not different among YC doses or starch level, but molar proportions of propionate, isobutyrate, and isovalerate were higher for HS than for LS. Mean ruminal ammonia concentration was increased in HS-fed heifers. Heifers fed HS had an increased number of viable, non-viable, and total fluid-associated bacteria, particle-associated bacteria, and total bacteria. Increasing YC dose linearly beyond 10 g d^-1 decreased viable and total fluid-associated bacteria. The effects of various YC doses on ruminal fermentation products, pH, and microbial total cell concentrations indicate diet dependency between source of readily available carbohydrates and YC addition in dairy heifers.